Switch BaseSessionService to use SequencedWorkerPool instead of FILE thread

chrome/browser/sessions/persistent_tab_restore_service_browsertest.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/sessions/persistent_tab_restore_service.h"
 
 #include "base/compiler_specific.h"
 #include "base/run_loop.h"
 #include "base/strings/stringprintf.h"
 #include "base/strings/utf_string_conversions.h"
+#include "base/threading/sequenced_worker_pool.h"
 #include "chrome/browser/chrome_notification_types.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/sessions/session_service.h"
 #include "chrome/browser/sessions/session_service_factory.h"
 #include "chrome/browser/sessions/session_types.h"
 #include "chrome/browser/sessions/tab_restore_service_factory.h"
 #include "chrome/browser/sessions/tab_restore_service_observer.h"
 #include "chrome/browser/ui/browser_window.h"
 #include "chrome/common/url_constants.h"
 #include "chrome/test/base/chrome_render_view_host_test_harness.h"
@@ skipping 129 matching lines @@
     SessionService* session_service = new SessionService(profile());
     SessionServiceFactory::SetForTestProfile(profile(), session_service);
 
     AddWindowWithOneTabToSessionService(pinned);
 
     // Set this, otherwise previous session won't be loaded.
     profile()->set_last_session_exited_cleanly(false);
   }
 
   void SynchronousLoadTabsFromLastSession() {
+    content::BrowserThread::GetBlockingPool()->FlushForTesting();
     // Ensures that the load is complete before continuing.
     service_->LoadTabsFromLastSession();
     base::RunLoop().RunUntilIdle();
   }
 
   GURL url1_;
   GURL url2_;
   GURL url3_;
   std::string user_agent_override_;
   scoped_ptr<PersistentTabRestoreService> service_;
@@ skipping 572 matching lines @@
 
   EXPECT_FALSE(service_->IsLoaded());
   TestTabRestoreServiceObserver observer;
   service_->AddObserver(&observer);
   EXPECT_EQ(max_entries, service_->entries().size());
   SynchronousLoadTabsFromLastSession();
   EXPECT_TRUE(observer.got_loaded());
   EXPECT_TRUE(service_->IsLoaded());
   service_->RemoveObserver(&observer);
 }